The ability of packages to protect substances they cover is highly dependent on the materials used to design and construct the package (reference: Food Packaging and Preservation; edited M. Mathlouthi, ISBN: 0-8342-1349-4; Aspen publication; Copyright 1994; Plastic Packaging Materials for Food; Barrier Function, Mass Transport, Quality Assurance and Legislation: ISBN 3-527-28868-6; edited by O. G Piringer; A. L. Baner; Wiley-vch Verlag GmBH, 2000). Preferred packaging materials minimize the penetration of moisture, light, and oxygen often referred to as barrier characteristics.
Many packages have been developed with enhanced mechanical barrier characteristics including greater tensile strength, burst strength, stiffness, tear strength, flex resistance, blocking, and coefficient of friction. These packages are made of polymer matrices that are mostly responsible for providing strength to withstand physical stresses such as pressure, temperature, shear forces, and moisture permeability (by preventing cracks in a package). Minerals, including calcium carbonate, may be added to polymer matrices to further enhance the mechanical properties of materials. Calcium carbonate is known to enhance shrink reduction and improved the surface finish of packages (Functional Fillers for Plastics; ISBN-13 978-3-527-31054-8; edited by Marino Xanthos; copyright 2005 WILEY-VCH Verlag GmBH). Also, calcium carbonate is known to reduce the cost of a package by being used in place of more expensive plastic resins.
Light barrier characteristics of materials are typically determined by opacity measurements. “Opacity” is defined as a material through which little or no light is able to pass and is determined by light transmittance measurements. A material having higher light transmittance values will be considered to have lower opacity and vice versa. Most food packages are developed to be poor light transmitters preventing light to contact food and prevent food spoilage. Calcium carbonate, when added to material having a polymer matrix, is traditionally viewed as having little effect on enhancing the material's opacity or decreasing its light transmittance. However, there are minerals, other than calcium carbonate, that when added to such materials are known to greatly enhance the opacity of the material. Titanium dioxide is such a mineral that greatly decreases the light transmittance of a material and is frequently used in food packaging.
Optical properties of calcium carbonate and titanium dioxide have been well studied and may be described using color mixing laws (Principles of Color technology; 3rd edition; Roy Berns; ISBN 0-471-19459-X; Copyright 2000, John Wiley and Sons. page 150) and the Kubelka-Munk theory (Kubelka, P. and F. Munk; Z. tech, Physik 31 (1930), 1-16). The Kulbelka-Munk theory teaches that opacity via visible light scattering, which is the function of titanium dioxide and calcium carbonate, is the result of refractive indices differences between the plastic medium and the mineral particles within the pigmentary size range of between 0.1 and 1.0 microns in diameter. A difference between the refractive index of the mineral blend and the plastic medium creates opportunities for opacity via light scattering. For example, the refractive index of calcium carbonate is near 1.59 at 547 nm and low density polyethylene (LDPE) is near 1.52 at 547 nm; delta of 0.7. Rutile titanium dioxide has a refractive index value of 2.61 at 547 nm and LDPE has a value of 1.52; difference of 1.09. Therefore, at equal concentrations, titanium dioxide provides an order of magnitude more opacity than calcium carbonate.
Packaging materials have been made that include mixtures of calcium carbonate and titanium dioxide. One example of such a material is described in a Japanese Patent Application No. Kokai 2013-252645 (P22013-252645A) teaching a package composition including calcium carbonate and titanium dioxide. Specifically, a method was taught of making a biaxial drawn multilayer polypropylene film for packaging having enhanced oxygen barrier properties. This multilayer package was described as having one polypropylene layer including from 3 to 15 mass % of calcium carbonate and from 0.5 to 3 mass % of titanium oxide. This patent application does not teach, or suggest, that calcium carbonate when added to a material will significantly increase the material's opacity. To enhance the shelf life of substances, including foods and pharmaceuticals, a need exists to identify new packaging materials having great light barrier characteristics enabling the protection of photo-oxidation sensitive entities present in such substances such as foods and pharmaceuticals.